Seasonal things happen periodically, during a certain season. In most stores, Christmas decorations and candy canes are seasonal items.
You'll probably find all kinds of fruit all year long in a large grocery store, but your local farmer's market most likely carries mainly seasonal fruit. This means they'll have apples in the fall, when they're in season, and strawberries in the spring. Workers are sometimes described as seasonal too — when they're employed just for the summer, for example: "The cafe hired lots of seasonal baristas in June."